ISSN 2305-6894

The effect of cathodic current density on the protective properties and cracking tendency in Co–Cr–W coatings

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1 Dmitry Mendeleev University of Chemical Technology of Russia, Miusskaya sq., 9, 125047 Moscow, Russian Federation
2 A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky pr. 31, 119071 Moscow, Russian Federation

Abstract: The paper shows that a change in the cathodic current density has a significant impact on the cracking tendency, through porosity and protective properties of coatings. As the cathodic current density increases, the ability to cover in cracks formed at the early stages of the process decreases during further electrolysis. It was established that the coating under investigation exhibits the best physico-mechanical characteristics when tungsten content is the highest. Moreover, in this case, we should expect an improvement in the corrosion properties. The highest hardness value is observed when tungsten content is high. The porosity of the coatings, determined by standard filter paper method, decreases when heat treatment is carried out for 2 hours at 200°C in air. It is shown that a change in current density within the range of 0.15–0.45 A/cm2 does not have a significant effect on the composition and hardness of the coatings. Long-term corrosion tests of the investigated Co–Cr–W coatings in simulated physiological medium (Hank’s solution) were carried out. Mass and deep corrosion indicators/readings and corrosion severity/resistance scores were calculated gravimetrically. The corrosion behaviour in Hank’s solution in the area of anodic potentials was studied. It is shown that the coating under investigation has an extended passive region/area, up to a potential of 1.2 V relative to a standard hydrogen electrode. The total/complete passivation current on the potentiodynamic polarization curve did not exceed 0.1 mA/cm2. The dissolution of the alloy, accompanied by the appearance of a yellow colour, occurred only at positive potentials exceeding ~1.3 V. This coating has a higher corrosion resistance in biological and chloride-containing media, compared to chromium coatings not (doped) with other metals (Ni, Co, Mo, W) or non-metals (P, B, C).

Keywords: corrosion inhibition, cobalt-chromium-tungsten alloy, stellite, hardness, current density, coatings composition, electroplating, corrosion in physiological media

Int. J. Corros. Scale Inhib., , 8, no. 4, 926-936 PDF (1 047 K)
doi: 10.17675/2305-6894-2019-8-4-8

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